A startup has built technology that could give any website or app the ability to recognize people’s faces, and even to identify their facial expressions. While some see the technology as creepy, Face.com, the company behind it, argues that most users don’t mind being recognized automatically online.

As computer vision software gets better, facial recognition software is becoming more common. With millions of images uploaded to services like Facebook every day, this technology can make social services more convenient, and could lead to completely new kinds of services and apps—but it also has obvious privacy implications.

For over a year, Face.com has made its technology available to software developers, enabling them to build it into a website or Web-connected apps. A website or app sends photos, which may be uploaded by users, to Face.com’s servers for processing and receives details that include the location of any faces, their gender, and whether they match other photos stored by Face.com. Last week, the service was upgraded to allow it to gauge a person’s mood, classifying them as happy, sad, surprised, angry, or neutral. It could already spot smiles, but has now gained the ability to classify whether a person’s lips are sealed, parted, or making a kiss. These new features could perhaps be used to automatically add more detailed tags to images or to challenge people to convey a certain mood with their expression.

Within three days of the launch of these new features, one website has begun using Face.com’s mood recognition feature. Moodbattle is a website that asks visitors with webcams to compete to pull the most extreme expressions associated with particular emotions (browse the results here).

“Last month, we processed more than two billion different photos,” says Face.com’s CEO Gil Hirsch, who adds that usage is growing. Some 20,000 developers have signed up to tap into Face.com’s technology; they can process 5,000 photos per hour for free, or pay for the ability to process more.

Hirsch says the four-year-old company recently became profitable, but he acknowledges that facial recognition still raises privacy concerns with many.

Last month, Facebook was forced to apologize to users after introducing a feature that uses facial recognition to suggest which of your friends appear in a photo, to speed up the process of tagging them. This is nearly identical to a Facebook app previously released by Face.com, although Hirsch declined to comment when asked if his company had supplied the  technology behind Facebook’s facial recognition.

Google also has sophisticated facial recognition software, but it has been careful to say it does not use it in the mobile app Google Goggles, which identifies objects snapped with a phone’s camera, or the recently launched Search by Image service.

Kelly Gates, a professor at University of California, San Diego, whose recent book examines how facial recognition technology is being developed, adopted, and understood, says concerns over the technology are largely due to an association with security and surveillance. Combined with the fact the technology interfaces with a very personal part of the body, “that means it is easy to write scary headlines about,” says Gates.

However facial recognition has clear benefits in a world of online socializing. “It seems that there is a need for technology that can help with that,” Gates says. “Now we are habituated to things like photo tagging it starts to seem attractive to automate it.” Gates thinks the usefulness of services like Face.com’s will be enough for most people to eventually accept facial recognition, just as they’ve accepted other technologies that initially caused privacy fears.

Hirsch argues that there has already been a shift in attitudes toward facial recognition. The technology has been accepted in desktop software for organizing photos from both Apple and Google, he points out. The key is to make sure that people feel in control of the technology, he says. For example, Face.com’s service can connect with Facebook to find your friends in a photo, but it will ask permission to access Facebook data and abides by a user’s privacy settings. As the technology appears in more places, people will warm to the notion of services that recognize them and their friends, says Hirsch.

Gates says that technology that recognizes moods or expressions is much less mature, and less accurate, than that which recognizes faces and applications for it are still unclear. Face.com’s mood recognition technology likely works best with posed expressions rather than natural ones, she adds, but that could still be useful: “It’s like emoticons, in that these are very simplistic expressions of emotions, but they can serve a purpose to communicate something.”